1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device that improves the issue of white washout relating to the off-axis viewing angle.
2. Description of the Related Art
Since LCD devices have the advantages of thin sizes and low power consumption, they are widely used as the screens of computers, portable electronic devices and televisions. Generally such an LCD device is constructed by a liquid crystal layer sandwiched between an upper substrate and a lower substrate, and then electrodes formed on the respective substrates are used to control the orientation of liquid crystal molecules for display.
Depending upon the brightness of environment, the so-called transflective LCD may function in two modes, including a transmissive mode utilizing the backlight and a reflective mode utilizing the incident light. The transflective LCD includes a transmissive area and a reflective area, in which the transmissive area is used in the transmissive mode, while the reflective area is used in the reflective mode. In the transmissive area, incident light coming from the backside of the display panel passes through the liquid crystal layer directly and then emits from the front side of the display panel. In the reflective area, incident light coming from the front side of the display panel passes through the liquid crystal layer and is reflected by a reflector, and then emits from the front side of the display panel.
Along with the expanding needs for small-sized LCDs used in devices like mobile phones, transflective LCD devices are required to provide higher image quality. For a conventional transflective LCD device, to make the respective gamma curves of the transmissive and reflective modes consistent, the liquid crystal cells in the transmissive and reflective areas are provided with varied cell gaps. However, this will make the manufacturing process more complicated. Further, because of the height difference in the boundary area, it is difficult to control the orientation of liquid crystal molecules with high accuracy. On the other hand, for applying different driving voltage levels for the reflective and transmissive modes, it requires to control the voltage by means of dividing pixels in the transmissive area, which causes the circuits to become complicated and reduces the aperture ratio as well.
FIG. 3 shows the gamma curve of a transflective LCD device disclosed in the JP Patent publication No. 2006-313358, wherein the pixels in the transmissive area are divided for voltage control. As FIG. 3 shows, compared to the gamma curve a, with a viewing angle of 0 degree at the front side, the gamma curve b, with a viewing angle of 45 degrees at the front side, is shifted. In this case, if pixels T1 and T2 in the transmissive area are provided with different signal voltages, the average luminance values from the viewing angle of 45 degrees at the front side will approach the gamma curve a, referring to the arrow in FIG. 3. As a result, the pixels in the transmissive area can be divided and thus provided with different signal voltages, making the average luminance values of this area approach those of the reflective area. Besides, the conventional LCD devices generally have the white washout problem relating to the off-axis viewing angle.
To overcome the white washout problem, two additional gate lines are arranged in the LCD device to control two different voltage levels to be applied to two pixel electrodes within a pixel unit. However, such construction will reduce the aperture ratio; moreover, when such construction is applied to a transflective LCD device, the corresponding data will not be written into the pixel electrodes in the reflective mode, which results in issues of white saturation and black saturation of the image.